1. Field of the Invention
This invention relates to a thermal printer, and in particular to a forward/backward printing thermal printer (referred to hereafter simply as a thermal printer) which moves a paper sheet forward and backward along a platen roller and prints on the paper several times so as to form an color image.
2. Description of the Prior Art
In a conventional thermal printer, a paper guided to a printing member while being gripped between a platen roller and pinch roller, and an ink sheet of a predetermined color guided to a thermal head by a different path to that of the paper, are superposed. Ink on the ink sheet is then transferred to the paper by the thermal head which is heated according to input image data. After printing with a first color ink sheet is complete, the colored ink sheet is discharged, and the paper alone is moved back to the position at which printing was begun. A second color ink sheet is then supplied, and image transfer performed using this second color ink sheet. The same paper is moved back and forth and the ink sheet is changed so that by superposing predetermined colors according to image data (usually, yellow, magenta, cyan and if necessary black), a color image composed of desired colors is formed.
FIG. 1 is a schematic view of, for example, the printing parts of a conventional thermal printer as is disclosed in, for example, Japanese Tokkai Sho 64-42263. This color thermal printer comprises a thermal head 12 which heats, according to input image data, ink on an ink sheet 10 consisting of a PET film or the like coated on one side with a thermofusible ink so that the ink melts and is transferred to a paper 11, a platen roller 13 which moves the paper 11 and ink sheet 10 on a printing member of the thermal head 12, a supply side pinch roller 14 which transports the paper 11 while pressing it on the platen roller 13, and a discharge side pinch roller 15. The paper 11 is firmly gripped between the platen roller 13 and the pinch rollers 14, 15 so that even if the paper 11 is moved forwards and backwards in order to perform printing by superposition, positional errors due to paper transport are avoided and color errors are prevented.
The action of a color thermal printer having this construction will now be described with reference to FIG. 2. As shown in FIG. 2A, the supply paper 11 is guided between the platen roller 13 and the supply side pinch roller 14 to the thermal head 12 or to a previously supplied ink sheet 10, and then passes between the platen roller 13 and discharge side pinch roller 15. An action referred to as blank feed is then performed. In this action, after the paper has been sent to the position shown in FIG. 2B, it returns to the position shown in FIG. 2A without any printing being performed. At this time, the pinch roller which is behind the paper with respect to its forward direction of movement, rotates at a slower speed than the platen roller. In other words, the pinch roller 14 on the paper supply side rotates at a slower speed than the platen roller until the paper reaches the position shown in FIG. 2B, and the pinch roller 15 on the paper discharge side rotates at a slower speed than the platen roller 13 until the paper returns shown in FIG. 2A. A back tension therefore acts upon the paper 11 when the paper is transported forwards and backwards due to the pinch roller 14 or 15, drawing the paper out so that it is in close contact with the platen roller 13.
After the action of blank feed, while the paper 11 is moving from the position shown in FIG. 2A to the position shown in FIG. 2B, the thermal head 12 which is in pressure contact with the platen roller 13 via the paper 11 prints in yellow according to yellow image data by using a yellow ink sheet 10 which supplies yellow ink.
Next, after releasing pressure contact of the thermal head 12 and the platen roller 13, the paper 11 is returned to the position of FIG. 2A, the ink sheets 10 simultaneously changes to the next color (magenta), the thermal head 12 again comes into pressure contact with the platen roller 13 and printing is performed in magenta. Subsequently, the same paper 11 is printed in the same way by ink sheet of desired colors, and a color image consisting of desired colors is formed on the paper 11.
However, in the heat transfer color thermal printer having the aforesaid construction, positional errors when the paper 11 is moved forwards and backwards must be prevented, and it is therefore necessary to ensure that the paper is constantly supported on the platen roller 13 by the pinch rollers 14, 15. In other words, when the paper 11 was sent for printing, paper feed had to be stopped at a position where the rear edge of the paper 11 could be gripped between the platen roller 13 and the pinch roller 14 as shown in FIG. 2B, and when the paper 11 was returned to be printed by the next color, paper return had to be stopped at a position where the front edge of the paper 11 could be gripped between the platen roller 13 and the pinch roller 15 as shown in FIG. 2A. If the paper 11 does not return properly when it is attempted to return it to its position between the pinch rollers and the platen roller after being released from these rollers, printing by superposition cannot be performed, the paper creases, and goes slack which leads to a color error. It was therefore impossible to release the edges of the paper from the rollers' grip. As a result, printing could not be performed on the parts of the paper extending from the contact points of the pinch roller and the platen roller to the thermal head, corresponding to a distance L.sub.1 in the case of the front edge of the paper, and a distance L.sub.2 in the case of the rear edge of the paper, and the length of paper over which printing could be performed was consequently reduced.
In order to solve the aforesaid problems, as shown in FIG. 3, a thermal printer wherein a paper 31 supported by a clamp 32 is printed with a predetermined color while the paper 31 is directly transported in the direction X in the figure via a predetermined path by a paper transport mechanism 33, and wherein printing can be performed up to the rear edge of the paper, is disclosed in for example Japanese Tokkai Hei 4-10962. In this thermal printer 30, the paper 31 is transported while being supported by a guide means 35 so that, even after the rear edge of the paper 31 has passed the contact point of a platen roller 34 and a pinch roller 37, paper transport continues and printing can be performed up to the very edge of the paper 31. After printing of the first color is completed, a thermal head 39 withdraws to the position shown in FIG. 3B, a drive gear 36b simultaneously rotates in the direction R.sub.1 shown in the figure, and a fan-shaped gear 36a rotates in the direction R.sub.2 shown in the figure. The pinch roller 37 which is fixed to the fan-shaped gear 36a moves to a position where it can grip the paper 31 which has been printed up to the very edge. Subsequently, the platen roller 34 and the guide means 35 rotate in opposite directions to their rotation directions during printing, the clamp 32 moves in the direction X.sub.2 shown in the figure, inserts the paper firmly so that it is gripped between the platen roller 34 and the pinch roller 37, and the paper is then returned to the position where printing began.
After the backward movement of the paper 31 is complete, the drive gear 36a rotates in the opposite direction R.sub.1 so as to return the pinch roller 37 to its initial position (position in FIG. 3A), and the thermal head 39 comes into pressure contact with the platen roller 34 in order to begin printing with the next color. While the paper is returning as described hereintofore, an ink sheet 38 is changed over. The paper 31 is then overprinted by a linear back and forth motion in the same way, and a color image composed of desired colors can therefore be printed on the paper 31 up to the very edge of the paper.
However, although according to this method the paper can be printed up to the very edge of one end of the sheet, the front edge of the paper could not be printed as it was supported by the clamp 32. Further, the length of the paper transport mechanism 33 required to move the paper 31 back and forth must be equal to or greater than the length of the paper 31, which tended to make the size of the heat transfer color printer too large.